Preparation of fiber-metal composites by electrodeposition



PREPARATION lor l*.,IFn-)IETAL cour'isnn BY ELEcfnonsro'sInon rma u"""""""" n. ...I il.

rArOO 'IIIIIIIIIIIIIIIIIII I United States Patent O 3,498,890 PREPARATION F FIBER-METAL COMPOSITES BY ELECTRODEPOSITION Amarnath P. Divecha, Falls Church, Va., Paul J. Lare,

Bowie, Md., and Henry Hahn, Fairfax, Va., assignors to Melpar, Inc., Falls Church, Va., a corporation of Delaware Filed Mar. 27, 1967, Ser. No. 626,156 Int. Cl. C23b 7/00, 5/48 U.S. Cl. 204-3 11 Claims ABSTRACT OF THE DISCLOSURE Preparation of fiber-metal composites by electrodepositing the desired metal matrix on a cathode support, while maintaining the fibers Within the electrolytic bath such that they are entrapped in the depositing metal, resulting in consolidated structures that are non-porous and of substantially theoretical density,

BACKGROUND OF THE INVENTION The present invention relates generally to incorporation of whiskers in metals and metal alloys, and in particular to processes for consolidating high strength ceramic fibers in a desirable orientation or alignment in the metal matrix to provide metal-Whisker composites possessing antifrictional properties, and, with subsequent bonding, useful in high-strength applications.

The advent of high strength single-crystal fibers (whiskers), and the availability of sapphire and silicon carbide whiskers in ever-increasing quantities, have generated extensive interest, study and experimentation in the incorporation of whiskers in metallic matrices, so as to combine the mechanical properties of ductile metals with those of high-strength fibers, oriented at will to manifest reinforcement in the direction of the applied stresses.

A basic requirement of a successfully reinforced composite lies in its ability to transfer load from one Whisker to another. Another requirement is the attainment of suitable whisker-matrix interface bonding. Wetting of the fibers by the matrix is also necessary, and it has been found for example that pure molten nickel bonds to alumina fibers under prolonged contact (about 30 minutes), and that very small additions of chromium (approximately 1%) also enhances Wetting.

In the past, fiber-reinforced metal composites have generally been prepared by liquid infiltration-and solid state powder metallurgy techniques, yielding very small Composites and demonstrating little potential in terms of reliability or upscaling` In the liquid infiltration process,

metallized Whisker mats are immersed or otherwise brought into contact with the molten metals, frequently resulting in dissolution of the Whisker coating before complete encapsulation of the whiskers by the metal can occur. Accordingly, incomplete bonding may exist between fibers and metal matrix of the composite, as well as porosity of the final structure. l

Variations of the standard powder metallurgical techniques by Which to achieve Whisker-metal matrix composites have included cold or hot pressing of Whiskermetal powder mixtures, followed by sintering. Another, method involving pressing requires the-use of metallized Whisker mats, yarns, or other suitable shapes, which are electroplated and hot pressed into fully dense structures. These prior art methods which employ pressing, however, produce significant Whisker damage during compaction at high pressures. Low pressure compaction on the other hand results in porous structures which require sintering over long periods of time. In any case, the previous pressing techniques have in many instances failed to produce ice complete encapsulation of the individual fibers by the metal.

Still another prior art method of producing fiberreinforced composites comprises a powder metallurgy technique in which the metal or alloy to be reinforced is comminuted to a tine powder and mixed with the fibers, a plasticizer, and an alkaline earth metal, at room temperature. The mixture is thereafter kneaded to a plastic mass at room temperature extruded, dried, sintered, cooled, and resintered, the sintering steps accomplished in a flowing hydrogen atmosphere. In addition to the number of operations required, only very small specimens, of composites have thus been produced, although of relatively high tensile strength and with substantially predetermined fiber orientation.

A prior art fusion technique of preparing the Whiskerreinforced composites includes the same initial mixing step as employed in the powder metallurgy'technique described immediately above, except that the plasticizer is omitted. The mixture is dried at elevated temperature, comminuted, and heated to its fusion temperature for a short period, Although fewer steps are required than in the immediately preceding technique, this process again results in small composite specimens, along with some loss of integrity in the final structure as a result of an oxidation reaction.

One significant problem resides in the provision of methods by which to achieve effective consolidation of the fiber metal matrix, in relatively large scale units, prior to effecting a satisfactory bond between fibers and metal matrix.

We have found a method by which such consolidation is achieved in a relatively simple manner. Moreover, the fiber-metal composites produced by our method appear to have substantial utility as anti-friction coatings as well as to be capable of providing, with subsequent bonding by compaction or other conventional methods, reinforced high-strength matrices.

SUMMARY OF THE INVENTION Accordingly, it is a broad object of the present invention to provide improved processes for preparing and consolidating fiber-metal composites.

Briefly, according to the present invention the fibers are agitated within a portion of an electroplating bath maintained in a separate porous Crucible, into which the cathode of the electroplating equipment extends, apart from the main bath containing an anode of the metal or metals to be deposited. Agitation of the whiskers prevents settling thereof, and the porosity of the crucible permits entry of the metal ions into the cathode region while maintaining the whiskers in a limited area about the cathode. During electroplating, the fibers are entrapped in the depositing metal, forming composites of high density and consolidation and substantially non-porous character.

A feature of the present invention is entrapment of the fibers in the plating metal matrix during the deposition process.

Another feature resides in the operators ability to control the amount and orientation of fibers entrapped in the depositing metal by control of the fiber content of the bath, degree of fiber agitation, and current density.

Advantages of the processes in accordance with the present invention over those of the prior art include the fabrication of a non-porous, fully dense composite; absence of added pressures on the whiskers, thereby eliminating pressure-produced Whisker damage characteristic of some compaction methods; preparation of composite at or slightly above room temperature; and capability of forming complex shapes With relative ease, thereby eliminating damage to fibers which normally occurs during forging, rolling and other mechanical Working.

Accordingly, it is a more specific object of the present invention to provide processes for preparing fiber-metal composites wherein the fibers are entrapped in controlled quantity in a metal matrix electrodeposited on a support material, using standard electroplating equipment and procedures.

The invention is contemplated as being of significant utility in the formation of composite coatings upon conventional materials such as copper used in moving electrical machinery. In particular, fiber reinforced coatings may be formed Which enhance the surface properties of conventional materials in terms of, for example, frictional Wear. Thus, a composite of sapphire whiskers containing copper on high conductivity copper Would result in relatively little loss in electrical conductivity of the moving contact while significantly enhancing its frictional and Wear properties. A specific application of such a moving contact Would be in deep space and other applications where environmental factors preclude the use of conventional lubrication or destroy such surface films as normally enhance percussional couplings.

BRIEF DESCRIPTION OF THE DRAWING The above and still further objects, features, and attendant advantages of the present invention Will become apparent from a consideration of the following detailed description of a preferred exemplary process, especially when taken in conjunction With the accompanying drawing wherein The sole figure is a sectional schematic view of suitable dynamic bath electroplating apparatus.

DESCRIPTION OF AN EXEMPLARY PROCESS Referring first to the figure, the electroplating arrangement comprises a container for electrolyte solution or bath 12, into Which an anode composed of the metal to be plated projects. The electrolytic bath 12 is composed, in a conventional manner, of a solution of a salt of the metal being plated. Electrolyte 12 is stirred by the whirling blades 17 of rotating shaft 19 to prevent settling and/ or separation of constituents and to insure uniformity of bath temperature throughout the solution.

If desired, the temperature of bath 12 may be raised above room temperature (approximately 27 C.) by means of a heated surface 22 on which container 10 resides. Within container 10 there is placed a porous container or crucible 25, preferably of ceramic material, holding a high concentration 27 of whiskers to electrolyte, up to 50% or more by volume. This permits a controlled retention of the whiskers in a limited region about the cathode 30, a considerable economic factor in addition to uniformity of and control over amount of fiber content in the eventual composite. The porosity of crucible 25 should be such as to allow bath penetration While preventing whisker dissemination into the main portion of the bath.

Cathode 30 is preferably an inert electrode composed of a material supporting current flow but incapable of forming a strong bond with the metal to be deposited (or with the material of the fibers to be entrapped in the depositing metal). The cathode may be arranged to rotate While maintaining electrical contact (as by a sliding connection) with one terminal of the power source (not shown), the current leaving the electrolyte via that route, so as to provide agitation and recirculation of the Whiskerelectrolyte bath 27.`This, of course, prevents settling of the whiskers and any non-uniformiy of entrapment of the fibers during the electro-deposition process. Alternatively, stirring or other means of agitation of the whiskers may be accomplished by use of mechanical, ultrasonic or magnetic stirrers, or by bubbling air through that portion of the bath within crucible 25, so that a stationary cathode may be employed.

Electrodeposition of the metal from the anode and/or from the electrolyte solution is accomplished in a completely conventional manner, with replenishment of the strength of the electrolyte and/or renewal of the anode as necessary to effect the formation of a desired thickness or shape of the composite on the cathode. As the metal ions move through the bath and, via the pores in crucible 25, plate the cathode, the whiskers are entrapped in the depositing metal. Use of a rotating cathode forces the whiskers to flow across the surface thereof, yielding some degree of control of Whisker alignment in the deposit. After the composite is deposited, the cathode may be stripped therefrom by either a mechanical or a chemical method conventionally employed for removal of one material or composition from another.

Entrapment of whiskers in the deposit is controlled by degree of agitation, Whisker content of the bath, and current density.

The invention is applicable to all metals and alloys which can be electroplated from solutions which do not degrade the fibers. As specific examples, however, alumina fibers and silicon carbide fibers may be used, the anode may be composed of cast nickel, in which case the electrolyte solution may be nickel ammonium sulfate, and the cathode composed of aluminum. The -bers may be coated with an insulative material, such as silicon monoxide (SiO).

In particular, nickel AWas deposited using a cast nickel anode, nickel sulfamate electrolyte, A1203 (alumina) whiskers, uncoated and coated with nickel, and aluminum cathode, with bath temperatures ranging from room temperature to about F., plating current ranging from about 0.1 to about 1.0 amp., plating time ranging from about l0 to about 125 hours, Whisker sizes ranging from about 0.2 to about 30 microns, and Whisker concentrations of about 20 percent yby volume of electrolyte. The character of the electroplate Was almost uniformly a heavy smooth deposit in each case.

The electrodeposition process may be followed -by hot pressing to promote bonding, and the fiber-metal matrix composite may be mechanically rWorked by any ofthe conventional processes such as rolling, and additional bonding of metal matrix to the whiskers may be achieved by sintering at high temperatures, if desired.

We claim:

1. Process of forming a fiber-metal composite coating on a metal member, Which comprises electrodepositing the preselected metal for said composite from an elecrtolytic bath onto an electrode consisting of said metal member, and

simultaneously With said electrodepositing, agitating loose fibers to be contained Within said composite, in a restricted region of said bath separated from the lmain bath from which said metal is electrodeposited |by a porous Wall of porosity suiiicient to permit ions of said metal to reach said electrode and to prevent escape of said fibers from said restricted region to the main bath, said fibers being maintained in a suiiiciently low concentration in said restricted region of said bath to permit said agitation to maintain said fibers in suspension therein and Without producing significant fiber damage by compaction, whereby said fibers are entrapped in the metal plated onto said member.

2. The process of claim 1 wherein said iiber concentration in said bath is up to fty per cent by volume.

3. The process of claim 2 wherein said fibers are alumina.

4. The process of claim 3 wherein said fiber concentration in said bath is approximately 20 percent.

5. The process of claim 2 wherein said electrodeposition is followed by hot pressing the fiber-metal composite after a desired thickness of said coating is obtained.

6. A process of preparing 'a fiber-reinforced metal composite from an electrolyte solution containing a salt of the metal for said composite and into which an electrode composed of said metal and a further electrode on which Vsaid composite is t0 ybe formed extend, said process comprising introducing the iibers for said composite into said electrolyte solution in a limited region about said further electrode, said region separated from the first-named electrode and from the remainder of said electrolyte solution by a partition having a porosity sufficient to permit passage of ions of said metal therethrough while retaining said iibers within said region, said 'bers having a volumetric concentration within said region sutiicient to permit freedom of movement therein and less than that concentration which would result in signicant liber damage by compaction thereof, and

maintaining said iibers in motion in said region to prevent settling thereof while electrolytically depositing said metal on said further electrode to entrap said fibers in the metal being deposited.

7. The process of claim 6 wherein said bers are high strength ceramic bers.

8. The process of claim 6 wherein said bers are composed of a material selected from the group consisting of alumina and silicon carbide.

9. The process of claim 6 wherein said fibers are alumina fibers ranging in size Ifrom about 0.2 micron to about 20 microns, said first-named electrode is composed of nickel, said electrolyte solution includes nickel sulfamate, said further electrode is composed of aluminum, said concentration of fibers is approximately 20 percent by volume, and said electrolyte solution is maintained at a temperature ranging from about room temperature to approximately F.

10. The process of claim 8 wherein a plating current of up to approximately l ampere is passed through said electrolyte solution.

11. The process of claim 6 further including the step of stripping the liber-reinforced metal composite from said further electrode after completion of said deposition.

References Cited UNITED STATES PATENTS 2,391,206 12/ 1945 Van Der Pyl 204-16 3,046,204 7/ 1962 Barron 204-16 3,061,525 10/ 1962 Grazen 204-3 3,374,154 3/ 1968 McCutchen 204-3 JOHN H. MACK, Primary Examiner T. TUFARIELLO, Assistant Examiner U.s, C1. Xg. 204-16 

